In versions < 2008 (which, based on other comments to some of the answers, I believe you are running), the most efficient way is to keep it as a datetime type and use date math to avoid string conversions.
SELECT DATEADD(DAY, DATEDIFF(DAY, '20000101', purchase_time), '20000101')
FROM dbo.table;
EDIT
If you want the date only for display purposes, not for calculations or grouping, that is probably best handled at the client. You can do it in SQL simply by saying:
SELECT dt = CONVERT(CHAR(10), purchase_time, 120)
FROM dbo.table;
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